PORTLAND, ORE.
— A flotilla of 100 fishing boats, rafts, and kayaks crossed the Willamette River to a downtown park in Portland, Ore., the other evening to rally for the Pacific Northwest's reigning icon: wild salmon, now plummeting toward extinction due to development across much of the Columbia River basin.

It was a typical event for a "green" city that has one of the best records in the United States for recycling, reducing greenhouse-gas emissions, using alternative energy, and providing public transportation and bike paths.

But Portland's amenities – its natural setting along the Willamette River and its youthful techie vibe – are drawing a surge of new people, threatening to erode the very qualities that drew people here in the first place. As the US approaches 300 million people, that's the story of the nation as well.

In many ways, Americans have mitigated the impact of their increasing presence on the land. Since reaching the 200 million mark back in 1967, they have cut emissions of major air pollutants, banned certain harmful pesticides, and overseen the rebound of several endangered species. Despite using more resources and creating more waste, they've become more energy efficient.

The danger, experts say, is that the US may simply have postponed the day of reckoning. Major environmental problems remain, and some are getting worse – all of them in one way or another connected to US population growth, which is expected to hit 400 million around midcentury. Some experts put the average American's "ecological footprint" – the amount of land and water needed to support an individual and absorb his or her waste – at 24 acres. By that calculation, the long-term "carrying capacity" of the US would sustain less than half of the nation's current population.

"The US is the only industrialized nation in the world experiencing significant population growth," says Vicky Markham, of the Center for Environment and Population, a nonprofit research and advocacy organization in New Canaan, Conn. "That, combined with America's high rates of resource consumption, results in the largest ... environmental impact [of any nation] in the world."

The boomer challenge

The changing nature of the population also has environmental consequences.

"Today's baby boomers – 26 percent of the population – are the largest, wealthiest, highest resource-consuming of that age group ever in the nation's history, and they have unprecedented environmental impact," says Ms. Markham.

The generation's preference for bigger houses and bigger cars – and the proliferation of them – are gobbling up more resources and creating more pollution, according to a recent study by the Center for Environment and Population. For example:

•Land is being converted for development at about twice the rate of population growth. When housing, shopping, schools, roads, and other uses are added up, each American effectively occupies 20 percent more developed land than he or she did 20 years ago.

•Each American produces about five pounds of trash daily, up from less than three pounds in 1960.

•While the US is noted for its wide open spaces, more than half of all Americans live within 50 miles of the coasts where population density and its environmental impact are increasing.

That concentration poses special challenges for areas near the coast, like Portland, where land is rapidly being gobbled up. The city's population, which is now a bit over half a million, is fairly stable. But surrounding population pressures are great. The metropolitan area grew about 30 percent during the 1990s to just over 2 million. It's projected to grow to 2.6 million by 2010 and to 3.1 million by 2025.

Some groups worry that Portland's growth will undermine its environmental sustainability.

"Population pressures are overwhelming the Portland region's ability to absorb the influx of new people, fueling congestion and rises in land and housing prices," the ecological research group Environmental Tipping Points concluded in an analysis. "Portland's growth rate is twice the national average. With these challenges ahead, it remains to be seen whether this growth will threaten the very assets that Portland's progressive land-use planning policies have managed to protect so far."

But recent US history suggests there are reasons for hope.

It's no coincidence, for example, that the modern environmental movement began about the same time that US population ticked past the 200 million mark 39 years ago.

Stanford University professor Paul Ehrlich's controversial book "The Population Bomb" had predicted that humanity's numbers around the globe would overwhelm natural resources, especially food production, in a Malthusian catastrophe.

Things haven't turned out that badly, given the dire signs of distress in that era.

It was a time when "our nation awoke to the health and environmental impacts of rampant and highly visible pollution – rivers so contaminated that they caught on fire, entire towns built upon sites so toxic that the only recourse was to abandon them," recalled Environmental Protection Agency Administrator Steve Johnson in a May speech.

He was commemorating the 35th anniversary of the EPA by pointing to the Cuyahoga River in Ohio and Love Canal near Buffalo, N.Y. He might have mentioned that the bald eagle – the nation's symbol – was headed toward extinction as well.

"But looking back, we see much to celebrate," Mr. Johnson added. "Our air is cleaner, our water is purer, and our land is better protected."

Oomph behind environmental laws

Generally speaking, that's true thanks largely to such groundbreaking federal laws as the Clean Water Act, the Clean Air Act, the Endangered Species Act, the Safe Drinking Water Act, and the Toxic Substances Control Act. Bipartisan coalitions on Capitol Hill and presidents of both parties enacted those statutes. Making them work, to the extent that they have, has involved full-time activists, grass-roots efforts at the community level, and courts of law.

Increasingly, business is also getting involved.

In the current issue of Atlantic Monthly magazine, for example, Weyerhaeuser Co. – whose history has included bitter fights with environmentalists over clear-cut logging – is pledging to reduce its greenhouse-gas emissions to 40 percent less than what they were in 2000 by 2020.

"We will do this by harnessing the benefits of a renewable, natural resource – biomass – as fuel in the boilers that generate steam and electrical energy in our mills," says Ernesta Ballard, senior vice president for corporate affairs.

Weyerhaeuser, based in Federal Way, Wash., is one of 41 corporate members of the Business Environmental Leadership Council, most of them Fortune 500 companies, including such familiar names as Boeing, DuPont, Hewlett-Packard, Intel, and Lockheed Martin. The group focuses on practical steps to reduce global warming.

In New Haven, Conn., last week, a program to educate corporate board members on the potential liabilities and opportunities tied to climate change was launched by Yale University, Marsh (a leading risk and insurance services firm), and the Ceres network of investment funds, and environmental and other public interest groups. The first training session will involve some 200 board members of Fortune 1000 companies.

Faith groups, including typically conservative evangelicals, have also taken up "creation care" through such efforts as the National Religious Partnership for the Environment. The coalition includes the US Conference of Catholic Bishops, the National Council of Churches USA, the Coalition on the Environment and Jewish Life, and the Evangelical Environmental Network.

Among other things, they're providing literature on the environment to parishioners, providing sermons to pastors, organizing "Earth Day" and other events, and going "green" in their own facilities.

Meanwhile, state and local governments in many ways have pushed well ahead of Uncle Sam in working to protect an environment from a population that is growing in both numbers and affluence. For example, 10 states have adopted the "Clean Cars Program," to reduce global warming emissions by 64 million tons by 2020.

At last count, 295 mayors (representing some 49 million people) have accepted Seattle Mayor Greg Nickels's "Kyoto challenge," modeled after the Kyoto treaty that the US didn't sign. The goal is to cut carbon-dioxide emissions in their cities to 7 percent below 1990 levels by 2012.

"All over the country in one way or another, communities are coming up against the issue of sustainability with their populations and their consumption styles," says Martha Farnsworth Riche, former director of the US Census Bureau.

Of all parts of the country, Portland and the Northwest generally come closest to addressing the issue. Oregon launched formal recycling with its bottle bill in 1971, the nation's first container-deposit law. It was one of the first states (along with Vermont) to enact statewide land-use planning in the early 1970s. Early on, it protected beaches from commercial development. For years, Portland has had model public transit, including a light-rail system that recently celebrated its 20th anniversary.

"At its root is a strong appreciation of the place we are among those who've lived here and those who come here," says Portland City Commissioner Dan Saltzman. "Those values get carried forward in public policies with great support of our citizenry and our business community.

"There's just a tremendous desire to try to avoid many of the pitfalls that we've seen other cities find themselves in, and on a more global perspective how to live more lightly on the land," says Commissioner Saltzman, who holds environmental engineering degrees from two universities.

As US laws and American attitudes toward energy and the environment have advanced, some experts argue, efficiency gains have outstripped population growth and consumption.

"The average new house today is about a third larger than the average house in 1970, however the energy consumption is about the same as the smaller house in 1970," says Steven Hayward, author of the 2006 Index of Leading Environmental Indicators, released this summer by the Pacific Research Institute in San Francisco and the American Enterprise Institute in Washington. "That's from insulation, new appliance standards, and so forth."

New houses may be more efficient, but their environmental impact grows in other ways.

"They use more resources to build and use," says Markham of the Center for Environment and Population. "Also, the average amount of land around houses is growing."

Lots of land to handle growth, some say

Some observers aren't that worried. "We're a very big country in terms of our land and our expansiveness," says demographer William Frey of the University of Michigan and the Brookings Institution in Washington. "The people who argue that we're going to run out of energy, that we're going to run out of water, that we're going to run out of other natural resources, overlook the fact that time and again technology has been able to overcome those limitations."

Even so, the US may face a stiff challenge in dealing with the environmental impact of its growing population.

Earlier this year, researchers at Yale and Columbia universities constructed an "environmental performance index" comparing 133 countries on the basis of environmental health, air quality, water resources, biodiversity and habitat, productive natural resources, and sustainable energy. The US ranked 28th. (New Zealand, Sweden, Finland, the Czech Republic, and Britain were the top five.) Among 29 Organization for Economic Cooperation and Development nations, the US ranked 23rd.